It is well known that ultrasonic signals may be used as a means of measuring wind speed and direction. Indeed, ultrasonic air flow meters have a number of advantages over mechanical air flow meters. Many of these advantages are a consequence of the fact that an ultrasonic air flow meter has no moving parts. The absence of moving parts reduces the number of components requiring maintenance and repair, thus increasing the life of ultrasonic air flow meters whilst decreasing their operating costs.
An example of a prior art arrangement which utilizes ultrasonic signals for the purpose of measuring wind speed and direction is illustrated in U.S. Pat. No. 5,343,744. The embodiments described in U.S. Pat. No. 5,343,744 employ three or more ultrasonic signal paths and three or more electroacoustic transducers. Each transducer has a directional response characterized by a primary lobe disposed on a primary axis, and a secondary lobe centered on the primary axis but inclined at a lobe angle. The second lobe surrounds the primary lobe in a conical configuration which diverges with distance from the transducer. Each transducer both transmits and receives firstly a continuous wave acoustic signal and then an acoustic pulse signal. The propagation time of a continuous wave signal may be measured with a high degree of accuracy, whilst the propagation time of a pulse signal may be measured rather less accurately, but over a greater air flow speed range. Thus, the embodiments described in U.S. Pat. No. 5,343,744 employ continuous wave signals and pulse signals to provide an air flow meter capable of accurately measuring air flow speed over a large range.
However, the use of both continues wave signals and pulse signals, each transmitted along three or more signal paths, requires the use of complex and costly processing circuitry. Furthermore, the high divergence of the transmitted signals is inefficient.
Providing an accurate measurement of gas flow velocities may also be difficult in practice. For example, in a single time of flight (ToF) arrangement in which the transducers are 30 mm apart, to provide an accuracy of 5 mm per second the ToF measurement needs to be accurate to within 2.5 nanoseconds. This would require a 400 megahertz clock and a signal with a phenomenal signal-to-noise ratio. Such an arrangement is just not practicable for reasons of cost and the amount of noise in any real environment.
Several air movement measuring devices have been described in the prior art U.S. Pat. No. 4,031,756 describes an ultrasonic air measuring device which is suitable for use with an aircraft.
U.S. Pat. No. 5,357,810 describes another ultrasonic transducer for air flow measurement.
The airflow measurement devices described in the aforementioned patents are costly, cumbersome to construct and operate and are not amenable to be hand-held during their use.
It is therefore an object of this invention to provide an airflow measurement device which is accurate and simple to use.
It is also an object of the present invention to provide an airflow measurement device which can be conveniently held by hand during measurement.
It is also an object of the present invention to provide an air flow meter capable of measuring the speed of an air flow by measuring the propagation time of pulse signals transmitted along two signal paths.
It is a further object of the present invention to provide an air flow meter employing transducers generating ultrasonic signals with low divergence in the vertical direction.